Polysilazanes and their derivatives are useful among other things, for the preparation of silicon nitride (Si.sub.3 N.sub.4). silicon carbide (SiC), Si.sub.3 N.sub.4 /SiC alloys, Si.sub.3 N.sub.4 /carbon alloys. Si.sub.3 N.sub.4 /boron nitride alloys, and mixtures thereof. These ceramic materials can be used as structural materials, protective coatings, and electronic materials because of their hardness, strength, structural stability under extreme environmental conditions and their wide variety of electronic properties. In particular, these materials can be formed into ceramic fibers of value for reinforcement of composite materials. See, for example, (a) Department of Defense Proceedings, Fourth Metal Matrix Composites Technical Conference, May 19-21, 1981, prepared for DOD Metal Matrix Composites Information Analysis Center; and (b) J. J. Brennan, "Program to Study SiC Fiber-Reinforced Matrix Composites". Annual Report to Dept. of Navy (Nov. 1980). Contract No. N00014-78-C-0503.
Historically, polysilazanes were first synthesized by Stock et al almost 60 years ago (see, e.g., Stock. A. and K. Somieski, Ber. Dtsch. Chem. Ges. 54:740 (1921)) via a simple ammonolysis technique (Scheme I). ##STR1## However, this approach usually produces mixtures of cyclomers where x is 3 to 5 that are obtained as the major products and small amounts of linear oligomers where y is less than or equal to about 10. Because of their low molecular weight, however, these linear oligosilazanes are too volatile to be used as preceramic materials.
In order to obtain higher molecular weight, nonvolatile materials, it was necessary to promote cross-linking reactions. In this manner, moderate molecular weight polysilazanes have been synthesized using a variety of techniques. See. e.g., Kruger, C. R. and E. G. Rochow, J. Polymer Sci. 2A:3179-3189 (1964). Rochow et al. discovered that ammonium chloride catalyzes cross-linking in simple oligodimethylsilazanee to form polysilazanes (Scheme II) which were proposed to contain cyclic monomer units cross-linked through nitrogen as suggested by the structure of Formula 1. ##STR2##
The Penn et al. work follows up on U.S. Pat. Nos. 3,853,567 to Verbeek and 3.892.583 to Winter et al., wherein a high temperature elimination/condensation reaction was shown to lead to soluble, highly cross-linked polymers as shown in Scheme III. Pyrolysis at high temperatures provides ceramic yields of 60% with a mixture of Si.sub.3 N.sub.4 and SiC ceramic materials. ##STR3##
A related cross-linking approach described, inter alia, in U.S. Pat. Nos. 4,312,970, 4,340,619, 4,535,007 and 4,543,344 begins with the preparation of tractable polysilazanes having Me.sub.3 Si groups in the polymer backbone (Scheme IV) with the highest molecular weights reported in the available literature, i.e. about Mw.about.15,000 D and Mz.about.39,000 D: ##STR4## Ceramic yields obtained from pyrolysis of this polymer are on the order of 45-55% with compositions of 96% Si.sub.3 N.sub.4, 2% carbon and 2% oxygen after curing.
U.S. Pat. No. 4,482,669 to Seyferth et al. discloses that it is possible to cross-link low molecular weight cyclic oligomers containing Si--H bonds adjacent to N--H bonds via the following reaction: ##STR5## The NH bond is catalytically activated by the strong base in this reaction. This type of cross-linking generates two-dimensional polymers, the solubility of which is limited by their sheet-like character. Ceramic yields of these materials are often quite high, up to about 86%. and typically provides Si.sub.3 N.sub.4 SiC and carbon in a mole ratio of 0.88:1.27:0.75. If the pyrolysis is carried out in an NH.sub.3 atmosphere, then the only product is Si.sub.3 N.sub.4 with the other products remaining as slight impurities.
Zoeckler and Laine in J. Org. Chem. (1983) 48:2539-2541 describe the catalytic activation of the Si--N bond and in particular the ring opening of octamethylcyclotetrasilazane and polymerization of the ring-opened intermediate. Chain termination is effected by introducing [(CH.sub.3).sub.3 Si].sub.2 NH as a coreactant giving rise to polymers (CH.sub.3).sub.3 Si-[NHSi(CH.sub.3).sub.2 ]n--NHSi(CH.sub.3).sub.3 where n may be 1 to 12 or more depending upon the ratio of the chain terminator to the cyclic silazane. The catalyst used was Ru.sub.3 (CO).sub.12. Other publications are as follows: W. Fink, Helv. Chem. Acta., 49:1408 (1966): Belgian Patent 665774 (1965); Netherlands Patent 6,507,996 (1965); D. Y. Zhinkis et al., Rus. Chem. Rev., 49:2814 (1980); K. A. Andrianov et al., Dok Akad. Nauk. SSSR, 227:352 (1976); Dok Akad. Nauk. SSSR 223:347 (1975); L. H. Sommer et al., JACS 91:7061 (1969); L. H. Sommer. J. Org. Chem. 32:2470 (1969); L. H. Sommer, J. Org. Chem. 32:2470 (1967): L. H. Sommer et al.. JACS 89:5797 (1967).
In general, control of the polysilazane molecular weight, structural composition and viscoelastic properties play a considerable role in determining the tractability (solubility, meltability or malleability) of the polymer, the ceramic yield, and the selectivity for specific ceramic products. In particular, the tractability plays a major role in how useful the polymer is as a binder, or for forming shapes, coatings, spinning fibers and the like. The more cross-linked a polymer is, the less control one has of its viscoelastic properties. Thus, highly cross-linked and low molecular weight polymers are not particularly useful for spinning fibers because the spun preceramic fiber often lacks tensile strength and is therefore unable to support its own weight. By contrast, high molecular weight, substantially linear polymers as provided herein are extremely important. Such polymers represent a significant advance in the art, as they provide chain entanglement interactions in the fiber-spinning process and thus enhance the overall tensile strength of the spun fibers.
An example of how molecular weight correlates with the properties of a particular polysilazane can be illustrated by the properties of H.sub.2 SiNMe].sub.x. The original synthesis of this material was reported by Seyferth et al. In Ultrastructure Processing of Ceramics, Glasses and Composites, Ed. Hench et al. (Wiley & Sons, 1984) via an aminolysis reaction: ##STR6## This method of preparation gives a mixture of a volatile cyclotetramer (35%) and nonvolatile oligomers. This mixture has an Mn of about 330 D and gives only a 28% is ceramic yield upon pyrolysis. Distillation of the volatile cyclomer yields 65% of low molecular weight nonvolatile oligomer (Mn=560) which is pyrolyzed to give a 39% ceramic yield. An improved method of preparing these oligomers is illustrated by Scheme VII: ##STR7##
By the method of this invention, working at temperatures of lower than about 0.degree. C. provides mostly nonvolatile linear oligomers (between about 85% and 95%) that require no distillation/purification step. For this product, the Mn is about 800-1.100 D (y.about.14-19). Pyrolysis of this improved oligomer gives significantly higher ceramic yields of 50% with some improvement in product quality, with Si.sub.3 N.sub.4 purities of above about 80%. the remainder being carbon.
By the method of this invention, the silazane product of Scheme VII can be further polymerized to give novel polymers with Mn greater than about 10,000 D, in some cases greater than about 20,000 D, Mw greater than about 16,000 D and in some cases greater than about 32.000 D, Mz greater than about 40,000 D and in some cases greater than 80,000 D, or with observable species having a molecular weight of higher than about 50,000 and in some cases higher than about 500,000 D. Molecular weights as high as 2,500,000 D (see Example 23) have been detected for the polysilazanes as provided herein. Pyrolysis of these true polymer species will give significantly higher ceramic yields than previously obtained, the ceramic yield to a large extent depending on the molecular weight distribution and the polymer processing. Si.sub.3 N.sub.4 purities of 80% or higher may be obtained, depending on the reaction conditions.
These novel high molecular weight polymers are soluble, exhibit a high degree of linearity and give higher ceramic yields and Si.sub.3 N.sub.4 purities than the oligomeric starting material. In addition, the viscoelastic properties of the novel compounds can be carefully controlled using the method of this invention. In particular, at higher molecular weights, these polymers exhibit non-Newtonian viscoelastic properties, allowing for chain entanglement which will increase the tensile strength required to draw the thin precursor fibers required to form ceramic fibers.
The high ceramic yields are of considerable value in binder applications, injection molded parts and in matrix applications. During pyrolysis the density/volume change from preceramic polymer (1-1.3 g/cc) to ceramic (3.2 g/cc for Si.sub.3 N.sub.4) can be significant. Thus, ceramic yields tar below theoretical will only magnify the resulting density/volume change. For example, a 50% ceramic yield for a Si.sub.3 N.sub.4 precursor of density 1.0 will result in a final decrease in volume of approximately 80%.
It should be noted that certain aspects of the present invention are discussed in copending application PCT/US86/00548, U.S. Ser. No. 908,685, filed Mar. 4, 1986, and the parent thereto, U.S. application Ser. No. 727,415, filed Apr. 26, 1985, now issued as U.S. Pat. No. 4,612,383. The disclosures of these related cases are hereby incorporated by reference in their entirety.